A process of fundamental importance in immunology is the binding of antigen to cell surface-associated immunoglobulin that leads to transmembrane signaling in specialized cells. Antigen-mediated crosslinking of immunoglobulin E-receptor complexes on mast cells and basophils leads to degranulation in the allergic response, while crosslinking of surface immunoglobulin on B lymphocytes by certain antigens is a primary singal for proliferation and differentiation into antibody secreting cells. The critical molecular features of the crosslinking events that lead to a bioloogical response in either of these systems are not understood, and the proposed studies are aimed at elucidating these features. In previous studies on the IgE-receptor system, fluorescence methods have been developed that have allowed the kinetics of binding and crosslinking by model bivalent antigens to e determined. These studies have revealsed that some bivalent antigens can crosslink IgE-receptor complexes very efficiently on the cell surface, yet trigger only a small cellulr degranulation response. Properties such as rate of crosslinking, crosslink lifetime, and rate of dissociation for these antigens will be compared with those of other bivalent antigens that trighger a much stronger biological response. By this means the relationship between these kinetic properties, activation, and inactivation (densensitization) signals will be investigated. Structural factors also play a role in determining the efficacy of crosslinking, and these will be studied using model bivalent antigens of different lengths. Distances between IgE-receptor complexes crosslinked by long, rigid bivalent antigens that trigger degranulation will be mapped by electron microscopic and resonance energy transfer methods. The methodologies developed on the IgE-receptor system will be extended to studies of model antigen binding and crosslinking of dansyl-specific surface immunoglobulin on B cells and hybridoma cell lines. These studies will determine whether the features of crosslinking that are critical for signal tranduction by IgE- receptor complexes on mast cells are also important for transmembrane signaling by surface immunoglobulin on B cells.